Organic electroluminescence elements (organic EL elements) have been used as self-luminous elements for image display devices, such as displays, and for surface light sources. Such an organic EL element (organic light-emitting diode (OLED)) is generally prepared by stacking a transparent electrode serving as an anode, an organic layer, and a metal electrode serving as a cathode in this order on a transparent supporting substrate such as a glass substrate or a transparent plastic film. Thus, upon application of a voltage between the transparent electrode and the metal electrode, electrons supplied from the cathode and holes supplied from the anode are recombined at the organic layer. Then, when excitons generated by the recombination undergo transition from an excited state to a ground state, EL emission occurs. Light of the EL emission goes through the transparent electrode, and then is extracted to the outside on the transparent supporting substrate side.
However, such an organic EL element has a problem that light generated at the organic layer cannot be extracted to the outside sufficiently. Specifically, there is a problem that a large proportion of light generated at the organic layer disappears as heat during repetition of multiple reflections inside the element, or propagates inside the element and exits from end portions of the element, so that a sufficient external extraction efficiency cannot be achieved. For this reason, improvement of the external light extraction efficiency by using a diffraction grating having a concavity and convexity shape formed therein and the like have been proposed recently in the field of organic EL elements.
International Publication No. WO2011/007878 (PTL 1) discloses an example of such an organic EL element using a diffraction grating having a concavity and convexity shape formed therein. This organic EL element comprises: a transparent supporting substrate (A); a cured resin layer (B) stacked on the transparent supporting substrate; and a transparent electrode (C), an organic layer (D), and a metal electrode (E), which are stacked in this order on the cured resin layer (B), wherein the cured resin layer (B) has concavities and convexities formed on a surface thereof, and the concavities and convexities have such a shape that when a Fourier-transformed image is obtained by performing two-dimensional fast Fourier transform processing on a concavity and convexity analysis image obtained by analyzing the shape of the concavities and convexities by use of an atomic force microscope, the Fourier-transformed image shows a circular or annular pattern substantially centered at an origin at which an absolute value of wavenumber is 0 μm−1, and the circular or annular pattern is present within a region where an absolute value of wavenumber is within a range of 10 μm−1 or less. The organic EL element described in PTL 1 has a sufficiently high light extraction efficiency, but there is a demand for development of an organic EL element which can achieve improvement in light extraction efficiency at a higher level.
Meanwhile, it is also known in the field of organic EL elements that even when light generated in an organic layer is extracted from a transparent electrode (for example, an ITO film), part of the light is repeatedly reflected in a transparent supporting substrate (for example, a glass substrate, or the like) present outside the transparent electrode, and is trapped in the transparent supporting substrate. In this respect, methods conventionally proposed for extracting such light trapped in a transparent supporting substrate include an approach in which the light is extracted by preventing the total reflection of the light at the interface between the transparent supporting substrate and the air by using a lens or the like; and the like. When an optical member such as a lens is used as described above, the light extraction efficiency is improved in conventional organic EL elements, but there is a demand for development of an organic EL element which can achieve improvement in light extraction efficiency at a higher level in a case where an optical member such as a lens is used in the same manner. Note that PTL 1 mentioned above does not describe any of the use of a lens for light extraction and the use of phosphorescent light emission for light emission of the element (an organic EL element using phosphorescent light emission).